A Closer Look At Minimalist Running Shoes
The science. Research has shown that barefoot running and some minimalist footwear (Vibram) promote a less dorsiflexed position of the foot at contact as well as a shorter stride length.2 This decreases the likelihood of heel striking. Studies showed the knee range of motion during stance was lower in barefoot gait and the knee was more flexed at contact than barefoot as well.2,3 Kerrigan’s study suggests that these kinematic changes reduce torque forces on the knee.4 Researchers have linked the avoidance of heel strike to reduced impact loading rate, which is also a proposed means of reducing overuse injuries. To date, however, there are no studies that validate claims of reduced injury risk.5
The dilemma. While many manufacturers claim their shoes promote a more natural running form, most lack validating research on their specific shoe models and simply refer to research on barefoot running, which is obviously not the same. In addition, while knee loading may be reduced, the eccentric loading and pre-activation of the triceps surae increase with barefoot running but not necessarily with all minimalist shoes.6,7 Many manufacturers also have running form coaching videos on their Web sites in order to teach runners how to use the shoes and/or how to transition to them from conventional shoes. A significant part of most of these coaching instructions is to reduce stride length and increase cadence. This form change alone, regardless of the shoe, will affect limb stiffness and impact loading rates for runners.
The question then arises: is it the shoe or the conscious running form change that is more important? (Running form changes are not without risk but discussion of that topic is beyond the scope of this article.) We cannot overlook that not all minimalist shoes elicit the same “natural” movement patterns that the barefoot condition does. Many runners are still able to heel strike and land with the knee more extended when wearing minimalist shoes in comparison to running barefoot.
Pertinent Insights On The ‘Zero Drop’ Phenomenon
The term “zero drop” refers to the relationship of the heel height to forefoot height. Conventional running shoes typically have a two-to-one differential with the heel often being 1 cm higher or more. Minimalist shoes have reduced this differential to varying degrees. Zero drop refers to a flat shoe construct with no difference between heel and forefoot. Some minimalist models have 4 to 8 mm of differential.
The marketing. Shoe companies contend that lower heel heights reduce knee loading, promote midfoot landing, are more “natural” and more comfortable.
The science. A study by Kerrigan and co-workers did show that loading of the knee is higher in shoes with elevated heels and authors have proposed that cushioned heels create a “pseudo-neuropathic” effect that makes heel strike painless.4,8 Heel striking in and of itself is not necessarily pathologic when running but hypothetically, heel striking combined with a longer stride and a more extended knee at contact may contribute to higher impact loading rates.
The dilemma. Even with a lower heel height, many minimalist shoes have cushioned midsoles that still allow runners to heel strike painlessly. In addition, the reduced heel height and mid- to forefoot landing pattern can increase the eccentric loading of the triceps surae.
While the force on the knee and loading rate may be less, this change does not automatically render the runner immune to injury. It essentially reduces the loading in one part of the limb and increases it in another.